Sunshine, sewage to power cities of the future

"These are the three giant stomachs of Lille."

Amid the hum of machinery and warm odour of putrefying autumn leaves, official Pierre Hirtzberger is explaining how three giant fermenters can convert household food waste, trimmings from parks and gardens and the slops from school and hospital canteens into enough methane gas to power about a third of the buses in the French city.

"The process is exactly the same as in the stomach of a cow," he said, gesturing towards three biodigesters which each hold 20,000 cubic metres of rotting liquefied waste.

"The objective is to fuel 100 of Lille's buses with this biogas, out of a total fleet of 350," said Mr Hirtzberger, head of the city's urban waste research and development.

From San Francisco to Malmo, Sweden, cities around the globe are preparing for a new imperative: to accommodate the mass of world population growth and thrive, without further accelerating the release of carbon dioxide that threatens their existence.

With half the world's population already living in cities and the urban population projected to reach almost five billion by 2030, it is not just growth that puts them in the front line of climate change.

Even if populations escaping drought migrate to urban centres, the fact that 60 per cent of the world's 39 largest metropoli are located in coastal areas puts the cities themselves at risk in future centuries, from rising seas. Sunshine, tech creativity and a clued-in population help widen the range of options for places like San Francisco - the first city to make it a crime not to compost food and waste in city bins, in a bid to cut landfill use to zero.

Plenty of money and abundant sun are allowing Abu Dhabi to showcase a futuristic eco-city: Masdar City is a vision of solar panels powering pilotless taxis and trams and feeding desalinated water to citizens and its verdant palms.

Such visions make dazzling prospectuses for those eyeing a market which analysts expect to be worth a record $200 billion next year, and sunshine will be a major source of clean power as the cost comes down to make it competitive with fossil fuels.

But for many cities, particularly older centres in gloomier climates, the reality will be more like Lille - distilling energy from the excrement of citizens, the waste from restaurants and the mountains of unsold sandwiches left in supermarket fridges at the end of each week.

Much of it will just be plain boring - pumping insulation foam into loft spaces and wall cavities, fitting double or triple glazing - the stuff that can keep small builders busy even if economic slowdown stalls grand construction projects.

In all, it will require myriad different approaches to whittle down society's impact on the planet.

Cities in France, Sweden, Australia and the US are looking at an exotic mix of energy sources, and their choices prove that what looks good in architects' promotional literature is not necessarily what works on the ground.

Even within cities, the density of solar generation will vary accordingly the value of land, he added.

In pricey central business districts, solar panels will be stacked on rooftops, but in the suburbs small-scale solar plants will help supplement households' own generation.